One of the final steps in completing an oil or gas well is establishing communication between the well bore and the formation behind the casing and cement sheath. To establish this communication, perforation tunnels are formed through the casing and cement sheath and into the formation. These tunnels are formed by either bullet or jet perforators.
In many instances, the perforation tunnels formed in the formation are unstable and can collapse. The collapse of perforation tunnels severely reduces the productivity of the well. Perforation instability can result from the crushing or cracking of an otherwise well-consolidated formation during the perforation process. Perforation collapse in a weakly consolidated or unconsolidated formation is to be expected.
Other conditions that lead to perforation collapse include various well treatments. One of the most damaging situations is caused by a matrix acidization performed through the perforations. Often the HF content of a mud acid will weaken the intragranular bond to the extent that the matrix becomes unconsolidated. In Berea sandstone for example, one hundred pore volumes of 10/3 mud acid is sufficient to render the rock unconsolidated. This process leads to the collapse of the perforation tunnels and severely reduces the well's productive capacity.